This invention relates to a method of protecting information stored in a memory device such as an EPROM.
The manufacturers of Digital Signal Processors (DSPs), as a service to their customers, will manufacture the DSP to operate with the customer's own proprietary programs. As a part of that service, the manufacturers attempt to protect the customer's proprietary information. In some cases, the proprietary information is stored external to the DSP in memories such as EPROMs, masked ROMs, CDs and even floppy discs. The DSPs are mass produced and are not unique to any particular customer. Any customer may develop a program to run on the DSP by using the DSP's unique OP-Codes. The concern that a customer has is that, after a substantial investment in time and money developing a proprietary program, a competitor or even a program pirate may obtain his program by purchasing a copy of it and a DSP and then extracting the program.
One way of protecting proprietary programs is to encrypt them prior to their being read. U.S. Pat. No. 4,764,959 disclosed a single-chip microcomputer with an encryption function that encrypts the contents of the memory for secrecy protection when the programs stored in a Read Only Memory (ROM) are read to the outside. However, it is quite simple to reverse engineer the ROM and determine the program by observing the state of each memory cell of the ROM.
Smart Card manufacturers also have developed many techniques for authenticating (a form of encryption) the validity of data stored in a smart card's memory. For example, U.S. Pat. No. 5,343,530 disclosed one method.